(i) Field of the Invention
The present invention relates to a variably sound-absorbing device which is inserted in the propagation path of elastic waves and the sound-absorbing characteristic of which can be electrically varied.
(ii) Description of the Related Art
A usual sound-absorbing device, for example, a muffler decreases sound power emitted through its outlet in the manner that a lining of sound-absorbing material is provided in a duct or a sound-absorbing material is inserted in the duct to damp propagated sound waves. Such mufflers include resistance type in which a porous layer or a fibrous layer is used as sound-absorbing material and reactive type (or resonance type) in which a honeycomb and a punched plate are combined with each other. The resistance type disperses energy based on the viscosity and the thermal conduction of the medium. The reactive type disperses energy with a loss due to surface friction and momentum according to the motion of the medium.
The sound-absorbing characteristic of such a sound-absorbing device as described above is determined by characteristics of the sound-absorbing material used in the device or the shapes and the dimensions of the honeycomb and the punched plate. It is therefore hard artificially to vary the sound-absorbing characteristic though it varies with a change in environment such as temperature or pressure.
If a sound-absorbing device is inserted in the propagation path of elastic waves and the sound-absorbing characteristic, that is, the reflection and transmission characteristics for elastic waves of the device can be controlled at will, such a device is applicable to audio devices, sound apparatus, soundproof equipment and so on. There are expected any applicable field.
For this purpose, the present inventor et al. had previously invented and filed "A method for controlling the modulus of elasticity of a piezoelectric material" Japanese Patent Application No.8-230491 not yet opened). In this method, a pair of electrodes is formed on a piezoelectric material and circuit elements are connected to the electrodes to vary the modulus of elasticity and the loss factor of the piezoelectric material. Characteristics such as the sound-absorbing characteristic can be controlled with variations of the modulus of elasticity and the loss factor.